Titanium disilicide (TiSi.sub.2) has two phases that include a low-resistance phase C54 and a high-resistance phase C49. In semiconductor applications, the C54 phase is preferred because of its lower resistivity that generally results in faster operating parts. One method of forming the C54 phase of titanium disilicide includes depositing a titanium layer over an exposed silicon surface. The surface is typically first annealed at 600.degree.-650 degrees Celsius (.degree.C.) to form the C49 phase of titanium disilicide.
After unreacted portions of the titanium layer are removed, the substrate is annealed again at a temperature of at least 800.degree. C. to form the low-resistive C54 phase of titanium disilicide. If the etch to remove the unreacted portions would not have been performed between the two thermal steps, a titanium disilicide region formed from part of a gate polysilicon layer may electrically short to a titanium disilicide region formed from part of an adjacent source or drain region.
Higher component densities require that regions become smaller and thicknesses become narrower and thinner. As the titanium layer becomes thinner, forming the C49 phase of titanium silicide and transforming the C49 phase into the C54 phase become more difficult. One problem is thermal agglomeration, which occurs when a titanium disilicide layer of relatively uniform thickness is converted to discrete islands of titanium disilicide. This results in a discontinuous titanium disilicide layer. Thermal agglomeration is more likely to occur as the titanium disilicide becomes thinner and as the width of source regions, drain regions, and gate electrodes become smaller. The electrical problems of thermal agglomeration include high resistivity due to a discontinuous titanium disilicide layer, high p-n junction leakage within source or drain regions, or lower gate oxide integrity for a titanium disilicide layer formed from a portion of a gate electrode.
A need exists to form a disilicide layer in the C54 phase using a single step without forming an electrical short. A need also exists to form a relatively thin silicide layer that does not have the problems of thermal agglomeration.